This parameter is determined from an overscan-subtracted calibration
lamp exposure in LSS mode. The total signal is integrated and
normalized by the exposure time and the total number of original CCD
pixels (keeping into account possible binning). More information
can be found in the <a href="http://www.eso.org/observing/dfo/quality/FORS2/qc/lamps_qc1.html">plot tutorial</a>.